1. Field of the Invention
This invention pertains generally to testing an optical system with an interferometer, and more specifically to methods by which a coherently illuminated optical system can be aligned within an interferometer being used for measuring or inspecting that optical system.
2. Description of the Background Art
Interferometers are often used for taking optical measurements on an optical system. The process of photo-lithography, for example, employs a variety of such optical systems which must be checked for errors and aberrations. In order to accurately perform these tests, it is critical that the components within the optical system be aligned using an interferometer. One form of interferometer is a phase-shifting point-diffraction interferometer (PS/PDI). The PS/PDI [1] generates a spherical reference beam by pinhole diffraction in the image plane of an optical system under test. A PS/PDI is shown in FIG. 1 being used with an optic under test and a CCD detector. A monochromatic beam is diffracted by an entrance pinhole spatial filter and then passed through a coarse grating beamsplitter placed before the image plane on the object-side (or alternately, the image-side) of the optic under test. The beamsplitter generates multiple focused beams that are spatially separated in the image plane. One of the beams from the test optic is allowed to pass through a large window called the test window, within a patterned screen that is herein referred to as a "mask".
The mask used may be either a transmissive mask or a reflective mask. In a transmissive mask the selected area of the test window contains transparent features, such as alignment marks, or may contain a fully transparent window. In a reflective mask, which is often used for EUV radiation, similar features or windows are selectively reflective. Use of the transmissive form of mask element is generally described and depicted herein, as it is easier to visualize and to understand; although either form of mask element may be used within the inventive method. The mask is located in the image plane and the beam passing through the test window is referred to as the test beam.
Any beam so "chosen" by the selective masking contains nearly identical aberration information about the optical system. A second beam from the test optic can be brought to focus on a reference pinhole smaller than the diffraction-limited resolution of the test optic, where it is spatially filtered to become a spherical reference beam covering the numerical aperture of measurement. A controllable phase-shift between the test and reference beams is achieved by a simple lateral translation of the grating beamsplitter. The test and reference beams propagate from the image plane to a detector where the interference pattern is recorded. The detector is positioned to capture the numerical aperture of measurement, and may be used with or without re-imaging optics.
The PS/PDI has been successfully used in the measurement of multilayer-coated, all-reflective extreme ultraviolet (EUV) optical systems, operating near 13-nm wavelength [2], where the fabrication tolerances are in the sub-nanometer regime [3]. Using pinholes on the order of 100-nm diameter, two-mirror optical systems with numerical aperture (NA) of 0.06-0.09 and system wavefront aberration magnitudes on the order of 1-nm rms have been measured. Two-pinhole null tests have recently verified the high accuracy (0.004 waves, or 0.054 nm rms within 0.082 NA) that is attainable with the EUV PS/PDI [4].
During the alignment process, the test window of the mask is normally positioned to be centered on the test beam focus when the reference beam is properly captured and centered on the reference pinhole. The test window width in the direction of beam separation should be less than the beam separation distance to minimize the undesirable overlap of the reference beam through the window. In the EUV application, with a typical beam separation of 4.5 .mu.m (27 times .lambda./NA), the window widths are chosen to be 4.5 .mu.m or less. An additional constraint may be imposed to achieve the complete separation of the orders in the Fourier domain of the recorded intensity image; here the window width must be limited to two-thirds of the beam-separation distance. [5]
Considering the small pinholes used in the measurement of high-resolution optical systems, alignment is the most challenging aspect of using an interferometer such as the PS/PDI. This fact is compounded in short-wavelength applications where the interferometer exists inside of a vacuum chamber and may be incompatible with other optical alignment strategies. While the test beam is typically easy to align through the large image-plane window, the reference beam should be positioned onto the reference pinhole to within a fraction of the focal spot diameter. The small size required of this pinhole attenuates the reference beam and narrows the "capture range" over which interference fringes are visible. Until the reference pinhole is within the focus of the reference beam, only subtle clues are available to guide the alignment. During fine alignment, once the beam has been captured, the intensity of the test beam remains fixed, and proper positioning can be judged by assessing the point of peak fringe contrast.